Enhanced oil recovery and waterflooding refer to the displacement of fluids from an injection well penetrating a crude-oil containing subterranean reservoir toward a production well penetrating the reservoir for producing crude oil. Many processes and compositions have been developed for improving the percentage of crude oil that can be produced in enhanced oil recovery and waterflood projects.
One problem encountered in waterflooding and enhanced oil recovery projects is that some reservoirs have highly conductive zones which accept disproportionate percentages of the crude oil displacing fluids. These conductive zones can be referred to as "thief zones". In some reservoirs, the fluid conductivities of various permeable strata of the reservoir are so different that a high percentage of the injected fluid will be conducted through only a small percentage of the volume of the reservoir. In other reservoirs, a high percentage of the injected fluid will be conducted through natural or man-made fractures or other voids defined by the formation in which the reservoir is located. This problem can significantly increase the quantity of treating fluid required to displace a high percentage of crude oil from a reservoir.
Various treating fluids have been injected into subterranean reservoirs surrounding both injection and producing wells to reduce the rate with which fluid can be displaced through highly conductive zones. These treating fluids are characterized in that they have low viscosities when injected into the reservoir and have constituents that react within the reservoir to increase the viscosity of the treating fluids. The conductivity of permeable strata can be decreased by introducing a fluid that will attain a viscosity of about 5,000 to about 10,000 centipoise (cp) and can be significantly decreased by introducing a fluid that will develop a viscosity of about 1,000,000 cp. The conductivity of fractures or voids can be decreased by introducing a fluid that will develop a minimum viscosity of about 1,000,000 cp and can be significantly decreased by introducing a fluid that will develop a minimum viscosity of about 10,000,000 cp.
Examples of treating fluids which have been disclosed for reducing the conductivity of subterranean reservoirs are aqueous solutions of either water-soluble silicates with acidic activators or water-soluble polymers such as lignosulfonates, polyacrylamides, polysaccharides and carboxymethylcellulose with water-soluble dichromate activators. Aqueous solutions of water-soluble lignosulfonates with activating mixtures of water-soluble dichromate and alkaline earth metal halides, nitrates and carbonates have also been disclosed.